For the purpose of merchandising a product or an item, a machine may present an advertisement to a user of the machine. For example, a web server may insert a banner ad (e.g., an advertisement shaped like a banner) into a web page before providing the web page to a web client for presentation to a user, where the banner ad is an advertisement for an item. The banner ad may be provided to the web server by a manufacturer of a product, and the web server may, for a fee, cause the banner ad to be presented to one or more users who view the web page. As another example, the web server may allow a seller of an item to write a description of the item and publish the description as a web page that advertises the item as being available for purchase. The item may be a particular instance of a product being merchandised.